Cryogenic heat exchanger are attractive design alternatives from the standpoint that they do not use environmentally damaging refrigerants, but instead use a cryogenic heat transfer fluid such as a liquefied atmospheric gas.
Previous work in this area does not address the issue of making efficient use of cryogens. In many cases, the temperature and energy requirements of the cryogen and/or other coolant fluids, heat exchanging apparatuses and heat storage apparatuses do not match, thus causing inefficiencies in the freeze drying method and apparatus.
There has been an attempt to ensure the equal heat distribution in the water-ice condenser which leads to the freeze drying chamber. In U.S. Pat. No. 5,456,084 to Ron Lee, an attempt is provided for a cryogenic heat exchange system in which water-ice build-up on a condenser heat exchanger surface employed in the cryogenic heat exchanger system is more uniform as compared to that of the then prior art heat exchangers which utilize a cryogenic heat exchange fluid. In that sense, attempts were made to provide better control over the temperature in which the heat transfer using the cryogenic heat exchanger system takes place.
In U.S. application Ser. No. 08/709,027 filed Sep. 6, 1996 entitled "Method and Apparatus for Controlling Freeze Drying Process", which is incorporated herein by reference, there is provided a method and process which utilizes a single heat exchanger, cooled by a cryogenic refrigerant, to deliver cold heat transfer fluid directly to a condenser and, independently, to a freeze dryer or other refrigeration system, either directly or through a heater circuit, for cooling or heating the freeze dryer.
Notwithstanding the above, there is a need in the art for a method and apparatus to refrigerate the chamber shelves and water condenser of a freeze drying chamber utilizing a dispensable cryogen (primarily liquid nitrogen) and to allow the exhaust/waste gas from the cryogen supply to exit from the system at the warmest temperature possible, while at the same time, accomplishing with minimal pumping energy thereby for completing each freeze drying cycle with minimal refrigeration cost.